Electrochemical processing of inorganic chemicals in electrolytic diaphragm cells for the production of other inorganic materials is well known. The electrolytic cell generally comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and a microporous diaphragm that separates the anolyte compartment from the catholyte compartment. Microporous diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
A non-limiting example of a diaphragm electrolytic cell is the electrolytic cell that is used for the electrolysis of aqueous alkali metal halide solutions (brine). In such an electrolytic cell, the diaphragm is generally present on the cathode used in the cell, and separates an acidic liquid anolyte from an alkaline catholyte liquor. The electrolysis of alkali metal halide brine generally involves introducing liquid brine into the anolyte compartment of the cell and allowing the brine to percolate through the brine-permeable microporous diaphragm into the catholyte compartment. The microporous diaphragm is sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions (during electrolysis) from the catholyte compartment into the anolyte compartment. When direct current is applied to the cell, halogen gas is evolved at the anode, hydrogen gas is evolved at the cathode, and an aqueous alkali metal hydroxide solution is formed in the catholyte compartment. In the case of aqueous sodium chloride solutions, the halogen produced is chlorine and the alkali metal hydroxide formed is sodium hydroxide. Catholyte liquor comprising alkali metal hydroxide and unconverted brine is removed from the catholyte compartment of the cell.
During electrolysis, it is not unusual for the diaphragm of a diaphragm electrolytic cell to develop perforations (holes), which allow too high a flow of liquid anolyte into the catholyte compartment if the driving force on the liquid anolyte remains constant. Additionally, perforations can occur during initial electrolytic cell assembly or when such cells are disassembled for repair. When the flow of liquid anolyte into the catholyte compartment is too high, the concentration of the principal product formed in that compartment is lowered, which results in increased costs for unit operations employed to work-up, purify and concentrate that product, as well as an increase in the amount and cost of recycling process streams from those unit operations. In the case of a chlor-alkali electrolytic cell, too high a flow of brine through the diaphragm is evidenced by lower than desired concentrations of alkali metal hydroxide and higher than desired concentrations of hypochlorite ion in the catholyte liquor. When such a condition exists, there is a need for means to increase the resistance to flow, i.e., decrease the flow of anolyte, through the diaphragm due to perforations that may have developed in the diaphragm.